1. Field of the Invention
This invention relates to digital video processing.
2. Description of the Prior Art
The transmission or storage of video signals commonly makes use of data compression, whereby a digital representation of a video signal is compressed so that it can be transmitted or stored at a lower bit rate, only to be decompressed on reception or retrieval. This conserves (expensive) storage or transmission bandwidth.
Data compression can generate compressed data streams with various bit rates. However, compressed video data can only be decompressed at the rate at which it was compressed. This means that a change in the compressed bit rate would require a compressed video sequence to be decompressed to the full video bandwidth, before being compressed again at the new data rate.
This is undesirable because of the hardware required to achieve this would make it impractical in most situations.
These problems are particularly severe in compression systems such as MPEG which use so-called groups of pictures (GOPs) in which one picture (an xe2x80x9cIxe2x80x9d picture) is compressed independently of the other pictures (so-called intra-frame or intra-field compression), and other pictures within the GOP are compressed by predicting their image content from one or more I pictures or from one or more xe2x80x9cPxe2x80x9d pictures, which in turn are predicted from one or more I pictures. Motion vectors may be used to help predict parts of the pictures from the correct parts of the I or P pictures. However, the motion vector generation process is very hardware-intensive.
Once again, in order to change from one GOP structure to another GOP structure requires a full decompression followed by a full recompression into the new GOP structure. Problems can also occur when a video data stream has to be switched from one compressed source to another, even if the bit rate does not have to be changedxe2x80x94for example during editing or on switching to advertisements during a broadcast programme.
This invention provides digital video processing apparatus in which an output compressed video signal is derived from an input compressed video signal;
the input compressed video signal representing successive input pictures, in which a subset of the pictures are derived from other input pictures in accordance with a repetitive group of pictures (GOP) structure, using input picture motion vectors;
the output video signal representing successive output pictures, in which a subset of the pictures are derived from other pictures in accordance with a repetitive group of pictures (GOP) structure different to or not picture-aligned with that of the input compressed video signal, using output picture motion vectors;
the apparatus comprising a motion vector processor for deriving the output picture motion vectors from the input picture motion vectors.
The invention recognises that instead of having to perform another full vector estimation process to change from one GOP structure or alignment to another, motion vectors for use in the output GOP structure can be derived from the motion vectors of the input GOP structure. In many cases, vectors needed in the output signal may not exist in the input signal, but these can (for example) be extrapolated from suitably chosen vectors provided for other pictures of the input signal.
The invention also provides a method of digital video processing in which an output compressed video signal is derived from an input compressed video signal;
the input compressed video signal representing successive input pictures, in which a subset of the pictures are derived from other input pictures in accordance with a repetitive group of pictures (GOP) structure, using input picture motion vectors;
the output video signal representing successive output pictures, in which a subset of the pictures are derived from other pictures in accordance with a repetitive group of pictures (GOP) structure different to or not picture-aligned with that of the input compressed video signal, using output picture motion vectors;
the method comprising deriving the output picture motion vectors from the input picture motion vectors.